Roberts, Caserio - Basic Principles of Organic Chemistry (2nd edition, 1977)
.pdfORGANIC
NOMENCLATURE
0rganic chemists, regardless of what languages they speak, can communicate with one another about their chemical work simply by writing equations
and structural formulas. But this is a slow process if the molecules are complicated, and is not well suited for conversation (try describing a structural formula of a complex molecule to someone). For more rapid and efficient communication we need to have names for compounds, and we should have every reason to hope that, after 100 years, the names now in use would be clear, unambiguous, easy to pronounce, easy to spell and to remember, as well as being amenable to arrangement in alphabetical order. But, even more, we should hope that the names of organic compounds would contain enough information so we could generate the proper structures from them, and conversely, if we know the structures then the system would have simple enough rules that we could construct universally recognized and accepted names.
Unfortunately, these splendid ideals have not yet been realized. A good part of the problem is that people are resistant to change and especially resistant to changes in names. To give an example, the carboxylic acid, CH,-CO,H, commonly is known as "acetic acid." The name arises from the Latin word acetum, for sour wine or vinegar, and acetic acid is the principal constituent, besides water, of vinegar. A similarly common compound is called "acetone" and, in the ideal world, acetone should be structurally re-
0
II
lated to acetic acid. But acetone is CH,-C-CH,, |
and the name arises only |
50 |
3 Organic Nomenclature |
because acetone is formed by strong heating of the calcium salt of acetic acid,
0 |
|
I1 |
+ CaCO,, a reaction that is of no cur- |
(CH3-CO,),Ca---+ CH,-C-CH, |
rent importance whatsoever. Better nomenclature systems use names based on the name of the hydrocarbon with the same number of carbons in the
longest continuous chain in the molecule. On this basis, CH,CO,H |
is related |
0 |
|
Il |
with three |
to ethane and is called ethanoic acid, whereas CH3-C-CH, |
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carbons is related to propane and called 2-propanone. |
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As far as possible, we shall use these names as our first choices, because organic chemistry is growing too fast to sustain the present chaos of nonsystematic nomenclatures in current use. You might well ask why nonsystematic names persist for so long. The reasons are complex and variable. Alchemists intentionally used abstruse names and symbolism to disguise what they really were working with. Chemical industry, especially in the drug area, has practiced much the same thing in using unintelligible trade names and codes for proprietary products. Obviously, everyone who handles or sells chemicals is not a chemist, and to the nonchemist, a short nonsystematic name will make more sense than a longer systematic name. A salesman who markets tons of "acrolein," CH,=CH-CH=O, has little reason to adopt the systematic name, 2-propenal.
People probably persist in using nicknames for chemicals for much the same reason that they use nicknames for people. Nicknames are less formal, usually shorter, and imply familiarity with the subject. Another cogent reason to resist dramatic changes in chemical nomenclature is that it would make the current and earlier literature archaic or even unintelligible. Universal adoption tomorrow of a nomenclature system different from the one we use here would render this book instantly obsolete. As a result, changes usually are made in small steps and may not be really effective until a generation or more passes. (Consider in this context the efforts to convert monetary systems and weights and measures to the decimal system.)
Ideally, every organic substance should have a completely descriptive, systematic name to permit only one structural formula to be written for it. This ideal has been approached closely in some of the current nomenclature systems but, unfortunately, truly systematic nomenclature for very complicated compounds is often hopeless for conversational or routine scriptorial purposes. As a result, we will at times resort to using (common) trivial names, especially if it is impractical to do otherwise. Clearly, the description
9-(2,6,6-trimethyl-l-cyclohexenyl)-3,7-dimethyl-2,4,6,8-nonatetraen-l-olhas phonetic disadvantages as a handy name for vitamin A:
3-1 Alkanes |
511 |
A very important consideration for becoming more familiar with the systematic names is their increasing use in indexing systems. When organic chemists dealt with relatively few compounds, it was possible to accommodate a wide variety of special nomenclature customs. However, the rapid growth of knowledge in the past twenty years, which probably has doubled the number of organic compounds, has also enormously increased the burden on those who dedicate themselves to making this knowledge easily available to others by indexing the current literature. A natural reaction is to discard common names in favor of more systematic ones and to develop numerical designations suitable for computer processing. The difference in sizes of the Chemical Abstracts1 indexes for the years 1907-19 16 and for the current year should be convincing as to the need for having systematic names become more widely used and important. But the fact remains that the naming systems used in indexing are not always the same as those used in practice, and we are left with the necessity of having to know both.
Learning the nomenclature of organic compounds has many of the elements of learning a language, be it Latin or Fortran. Fortunately, like a language it does not have to be learned all at once. One can become familiar with naming of simple hydrocarbons, then study their chemistry (avoiding that part which involves compounds with as yet unlearned names), proceed to the naming of alkenes, study their chemistry, and so on. This is a very simple and natural way but can be inconvenient in a textbook if one wants to review the nomenclature of more than one class of compounds at a time.
In this chapter, we consolidate the nomenclature of a number of classes of compounds-an undertaking that may not seem very logical to someone who will soon be troubled enough with the chemistry of these compounds let alone their names. We recommend, however, a thorough study now of alkane and haloalkane nomenclature (Section 3-1) followed by a more cursory examination of the rest of the chapter. Then, as unfamiliar names arise, you can quickly review the basic rules for alkanes and proceed to the new class you have encountered. The idea is to have many of the important rules in one place. Nomenclature rules for other types of compounds are given in Chapter 7.
ALKANES
The most definitive set of organic nomenclature rules currently in use were evolved through several international conferences and are known as the International Union of Pure and Applied Chemistry Rules (IUPAC rules). We first shall describe this system for naming the hydrocarbons known as alkanes-
'A weekly publication of the American Chemical Society; an index to, and a digest of, recent chemical publications. The index for the ten-year period, 19071916, came to a total of 6700 pages. The index for the single year, 1975, amounted to 24,000 much larger pages !
52 |
3 Organic Nomenclature |
the so-called saturated paraan hydrocarbons that have no double or triple bonds, or rings, and conform to the general formula C,H,,+,.
The alkanes are classified as "continuous chain" (that is, "unbranched") if all the carbon atoms in the chain are linked to no more than two other carbons; or "branched chain" if one or more carbon atoms are linked to more than two other carbons:
CH3-CH2-CH2-CH2-CH2-CH,
continuous-chain hydrocarbon
CH3 CH3 |
CH3 |
|
I |
I |
I |
CH,-C-C-CH3 |
CH3-C-CH2-CH3 |
|
I |
I |
I |
H |
H |
CH3 |
branched-chain hydrocarbons
The first four continuous-chain hydrocarbons have nonsystematic names:
CH4 |
CH3-CH3 |
CH3-CH2-CH3 |
CH3-CH2-CH2-CH3 |
methane |
ethane |
propane |
butane |
The higher members, beginning with pentane, are named systematically with a numerical prefix (pent-, hex-, hept-, etc., to denote the number of carbon atoms) and with the ending -ane to classify the compound as a paraffin hydrocarbon, as in Table 3-1. T o specify a continuous-chain hydrocarbon, the prefix n- (for normal) sometimes is used. However, in the absence of any qualifying prefix, the hydrocarbon is considered to be "normal" or unbranched and we shall not use this prefix henceforth. You should memorize the names up
to CIOH22.
CH3-CH2-CH2-CH2-CH3
pentane (n-pentane)
The possibility of having branched-chain hydrocarbons that are structural isomers of the continuous-chain hydrocarbons begins with butane (n =4). The IUPAC rules for the systematic naming of these hydrocarbons follow.
1. The longest continuous chain of carbon atoms is taken as the parent hydrocarbon and is the framework on which the various substituent groups are attached. Thus the hydrocarbon 1 is a substituted pentane rather than a substituted butane because the longest chain has five carbons:
(gray area encloses longest chain of successive carbon atoms)
2,3-dimethylpentane
1
2. The substituent groups attached to the main chain are named by replacing the ending -ane of the alkane by -yl. We then have the allcyl groups
3-1 Alkanes |
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Table 3-1 |
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The Normal Alkanes, C,H, |
+, |
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n |
Name |
Formula |
n |
Name |
Formula |
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methane |
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undecane |
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ethane |
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dodecane |
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propane |
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tridecane |
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butane |
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tetradecane |
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pentane |
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pentadecane |
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hexane |
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eicosane |
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heptane |
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triacontane |
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octane |
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tetracontane |
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nonane |
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pentacontane |
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decane |
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hectane |
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(or alkyl radicals), the simplest examples being the methyl (CH3-) and ethyl (CH,CH,-) groups.
3. The parent hydrocarbon then is numbered starting from the end of the chain, and the substituent groups are assigned numbers corresponding to their positions on the chain. The direction of numbering is chosen to give the lowest numbers to the side-chain substituents., Thus hydrocarbon 1 is 2,3-dimethylpentane rather than 3,4-dimethylpentane. The prefix disignifies that there are two identical substituents:
|
T H 3 |
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'CH, |
C H 3 4CH2 |
CH3 |
I |
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*CH2 |
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I |
I |
I |
I |
TH3--$H-$H-CH3 |
not $H3-YH-$H--CH, |
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The prefixes used |
to designate the number of substituents follow up to ten. |
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1 |
mono- |
6 |
hexa- |
2 |
di- |
7 |
hepta- |
3 |
tri- |
8 |
octa- |
4 |
tetra- |
9 |
nona- |
5 |
penta- |
10 deca- |
|
2Confusion is possible when the numbering from each end is similar. T h e rule is that when the series of substituent locants are compared term by term, the "lowest" series has the lowest number a t the first point of difference. T h e compound
CH3-CH-CH-CH,-CH-CH,
I |
I |
I |
CH, |
CH, |
CH3 |
is 2,3,5-trimethylhexane, not 2,4,5-trimethylhexane.
3 Organic Nomenclature
Monois not used to designate a single substituent in systematic nomenclature, but may be used in conversation for emphasis.
4. Where there are two identical substituents at one position, as in 2, numbers are supplied for each, and the prefix, di-, tri-, and so on, is included to signify the number of groups of the same kind:
CH,-C-CH |
-CH1 |
2.2.3-trimethyl butane. 2 |
5 . Branched-chain substituent groups are given appropriate names by a simple extension of the system used for branched-chain hydrocarbons. The longest chain of the substituent is numbered starting with the carbon attached directly to the parent hydrocarbon chain. Parentheses are used to separate the numbering of the substituent and of the main hydrocarbon chain:
Additional examples of alkyl substituents and their names are listed in Table 3-2. These are further classified according to whether they are primary, secondary, or tertiary. An alkyl group is described as primary if the carbon at the point of attachment is bonded to only one other carbon, as secondary if bonded to two other carbons, and tertiary if bonded to three other carbons. Thus, if R is any hydrocarbon radical, the different kinds of alkyl groups are
RCH2- |
R2CH- |
R3C- |
primary (prim) |
secondary (sec) |
tertiary (tert) |
Confusion can arise here because we often refer to specific carbons rather than whole alkyl groups as primary, secondary, and so on. In this context, a carbon is primary if it is bonded to one other carbon, secondary if bonded to two, tertiary if bonded to three, and quaternary if bonded to four. Thus, either carbon of ethane is primary, the C 2 carbon in propane, CH,CH2CH3, is secondary, and the C2 carbon of 2,2-dimethylpropane, (CH3),C, is quaternary.
The situation with regard to naming alkyl substituents has been muddied considerably by the fact that the IUPAC rules allow use of trivial names for a few alkyl groups. Thus see-butyl sometimes is used in place of 1-methylpropyl, and tert-butyl in place of 1,l-dimethylethyl. These and other examples are included in parentheses in Table 3-2. Further odd-ball but less official customs are the prefix iso, which is reserved for substituents with two methyl groups at the end of an otherwise straight chain (e.g., isopropyl), and the prefix neo,
3-1 Alkanes
Table 3-2
Typical Alkyl Groups (C,H,,+,)
Primary (RCH,-) |
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CH3- |
CH3CH2- |
methyl |
ethyl |
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"\"3 |
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/CH-CH2- |
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CH3 |
butyl |
2-methyl propyl |
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(isobutyl) |
Secondary (R2CH-) |
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C'H, 1-methylethyl (isopropyl)
Tertiary (R,C-)
CH3
I
CH3-C-
I
CH3
I, I-dimethylethyl (tert-butyl)
CH3CH2CH2-
P ~ ~ 1 P Y
CH3
I
CH,-C-CH,-
I
CH3
2,2-dimethylpropyl
(neopentyl)
CH,
1-methylpropyl (sec-butyl)
CH,
1,I-dimethylpropyl (tert-pentylj
which is used to denote three methyl groups at the end of a chain (e.g., neopentyl, which is more properly called 2,2-dimethylpropyl). Also in common use are the names isobutane and neopentane for the hydrocarbons 2-methyl- propane and 2,2-dimethylpropane,respectively. There is no ambiguity involved in the use of iso and neo prefixes here, but the practice of using the name "isooctane" for 2,2,4-trimethylpentane is erroneous. Fortunately, use of these special names is declining.
H |
CH:, |
H |
CH, |
2-methylpropane |
2,2-dimethylpropane |
2,2,4-trimethylpentane |
(isobutane) |
(neopentane) |
(not isooctane) |
3 Organic Nomenclature
6. When there are two or more different substituents present, the question arises as to what order they should be cited in naming the compound. The system adopted by IUPAC and long practiced by Chemical Abstracts cites them in alphabetical order without regard for whether there is a multiplying prefix such as dior tri-. Examples are given below.
CW:,-CH2 CH3
I I
~H, - $ H2 - SH2 - $ H - ~H - $ H2 - ~HB
4-ethyl-3-methyl heptane (ethyl is cited before methyl)
4-ethyl-3,3-dimethyl-4-propyldecane
(ethyl is cited before dimethyl before propyl; when two chains of equal length compete for the main chain, preference goes to the chain with the greater number of substituents)
When a hydrocarbon is substituted with other than alkyl groups a new problem arises, which can be illustrated by CH3CH2C1. This substance can be called either chloroethane or ethyl chloride, and both names are used in conversation and in print almost interchangeably. In the IUPAC system, halogens, nitro groups, and a few other monovalent groups are considered to be substituent groups on hydrocarbons and are named as haloalkanes, nitroalkanes, and so on.
The alphabetical order of precedence is preferred for substituents of different types when two or more are attached to a hydrocarbon chain because this makes indexing and using indexes more straightforward:
I-chlorobutane (butyl chloride)
CH, |
CH, |
\ |
I |
CH-CH2-Br |
C1-CH-CH2-CH2-NO, |
/ |
|
CH, |
|
I-bromo-2-methylpropane(isobutyl bromide) |
3-chloro-I-nitrobutane |
The rest of this chapter is devoted to names of compounds we will not discuss for several chapters ahead and you may wish to stop at this point and return later as necessary. However, you should test your knowledge of alkane nomenclature by doing Exercises 3- 1, 3-2, 3-3, 3-9, 3- 10, and 3- 11.
3-2 Cycloalkanes |
57 |
Exercise 3-1 Draw structural formulas corresponding to the following names:
a. |
2,7,8-trimethyldecane |
c. 5-(1, I-dimethylpropyl)nonane |
b. |
2,3,4-trimethyl-4-propyl heptane |
d. 4-(chloromethyl)-5-(I-nitroethyl)decane |
Exercise 3-2 Give the IUPAC name for each of the following structures:
c. CH,CH,CHCH,CH(CH,),
I
Exercise 3-3 The following are improper IUPAC names. Determine what is incorrect or ambiguous about the name and give the correct name.
a. 2-methyl-3-propylpentane |
c. |
2,3,3,7,7-pentamethyloctane |
b. 3-methyl-3-chloropentane |
d. |
3-(1, I-dimethylethyl)pentane |
3-2 CYCLOALKANES
The cycloalkanes with one ring have the general formula C,H2, and are named by adding the prefix cyclo- to the name of the corresponding continuous-chain alkane having the same number of carbon atoms as the ring. Substituents are assigned numbers consistent with their posi in such a way as to give the lowest numbers possible for the substituent
e~~
1,4-dimethylcyclohexane |
1-ethyl-3-methylcyclopentane [not 1-ethyl-4-methyl- |
[not 3,6-dimethylcyclohexane |
cyclopentane because of lowest number rule, and not l-methyl- |
because of lowest number rule] |
3-ethylcyclopentane because of alphabetical order rule] |
The substituent groups derived from cycloalkanes by removing one hydrogen are named by replacing the ending -ane of the hydrocarbon with -yl
3 Organic Nomenclature
to give cycloalkyl. Thus cyclohexane becomes cyclohexyl, cyclopentane becomes cyclopentyl, and so on. Remember, the numbering of the cycloalkyl substituent starts at the position of attachment, and larger rings take precedence over smaller rings:
4 |
4 |
1-bromo-2-methylcyclohexane |
1-cyclopropyl-3-methylcyclobutane |
(2-methylcyclohexyl bromide) |
[not (3-methylcyclobutyl)cyclopropane] |
When a cycloalkane has an alkyl substituent, the compound could be called either an alkylcycloalkane or a cycloalkylalkane. The alkylcycloalkane name is the proper one:
(1, I-dimethylethyl)cyclopentane |
1-methyl-3-(I -methylethyl)cyclohexane |
or tert-butylcyclopentane |
or isopropyl-3-methylcyclohexane |
[not 2-cyclopentyl-2-methylpropane] |
[not 2-(3-methylcyclohexyI)propane] |
Exercise 3-4 Write structural formulas for each of the following:
a.(2,2-dimethylpropyl)cyclopentane
b.1,2,3-tri(chloromethyl)cyclopropane
c.1,4-dicyclohexy lcyclooctane
d.1-(1-methylcyclopropyl)-1,2,2,3,3-pentamethylcyclopropane
Exercise 3-5 Give the IUPAC name for each of the following: